1. Technical Field
The invention relates to a method for detecting flow-reducing changes in an exhaust-gas catalyst body of an exhaust-gas purification system during the operation of an associated internal combustion engine with at least one engine cylinder, the exhaust gas from which impinges upon the exhaust-gas catalyst body and to which an ion-current measuring probe is assigned.
2. Discussion
Generally, if exhaust-gas catalytic converters become too hot, they can lose not only their property of chemical conversion but can also suffer mechanical damage or even destruction. Fragments formed during this process can pass via the exhaust valves into an upstream engine cylinder and lead to engine damage. There is therefore a requirement to be able to detect those types of mechanical damage to the catalyst body which typically result in disintegration of its mechanical honeycomb structure and, associated with this, a reduction in the cross section of passage available for the exhaust gas flowing through.
One method that may be considered for detecting such mechanical changes in the exhaust-gas catalyst body which reduce the cross section of passage is to determine the exhaust-gas backpressure, which increases upstream of the catalyst body due to the said reduction in the cross section of passage of the latter. Due to the high temperatures prevailing, however, this procedure requires a relatively high outlay. Another possibility of detecting the mechanical state of the catalyst body is temperature measurement, but this necessitates a corresponding temperature sensor.
It is known practice to use an ion-current measuring probe in an engine cylinder, i.e. in its combustion chamber, to carry out ion-current measurement during a respective operating cycle of the engine cylinder, it being possible for a spark plug to act as the ion-current measuring probe. The ion-current signal supplied by the ion-current measuring probe is customarily used particularly for knock detection, see Patent Specification DE 196 14 288 C1, which also mentions that ion-current measurement can also be used to detect spark failures, which can cause damage to a connected exhaust-gas catalytic converter. This and further applications of ion-current measurement, such as ignition-point feedback, transient lambda-value control, injection control and exhaust-gas recirculation control, are mentioned in a magazine article with the title "Ion-gap sensing for engine control" in Automotive Engineering, September 1995, page 65.
The technical problem underlying the invention is to provide a method of the type stated at the outset by means of which flow-reducing changes in an exhaust-gas catalyst body, particularly those due to destruction of its mechanical structure by excessive temperatures, can be detected reliably with a relatively low outlay.